1. Field of the Invention
The present invention relates to multifocal lenses and especially aspheric multifocal lenses used to correct vision in patients having vision defects such as presbyopic vision, accommodative insufficiency, aphakia, or accommodative convergence defects.
2. Background
Existing multifocal lenses can be broadly categorized as either simultaneous or alternating designs. Alternating designs require lens translation to present the eye alternatively with distance and near lens segments. Simultaneous designs do not require lens translation but rather involve focusing both the distance and near images at the same time. Concentric simultaneous designs may have either a circular near segment surrounded by an annular distance segment (center-near) or the reverse (center-far). The optical performance of the concentric simultaneous designs is affected by changes in pupil size and by variations in lens position. The sensitivity to pupil size may affect the relative bias between distance and near, the optimum vergence, and the range of focus.
Aspheric multifocal lenses are a type of concentric simultaneous lens having an increasingly flatter curve from the center to the edge of the lens. This aspheric curve may appear on the posterior (ocular) side of the lens or on the anterior side of the lens, although posterior aspheric designs are the designs of particular interest in considering the present invention. The flattening of the posterior surface on these lenses produces the multifocal power in conjunction with the refractive index difference between the tear film and the lens material.
Good centering is essential if maximum visual acuity is to be achieved with the aspheric multifocals. Centering is enhanced by selecting lens base curves which are much steeper than the curvature of the cornea being fitted. However, while the optical performance of a lens fitted in this way is good, the potential for corneal edema is increased. Accordingly, optimal centering of current posterior aspheric designs must be compromised by fitting the lenses less steeply and by increasing the overall lens size. Due to the steepness with which the lenses are fit, a relatively flat spherical curve is often added to the edge of the posterior surface of the lens to improve tear exchange under the lens.